This invention relates generally to the joining of electrical contacts or connectors to circuit boards, and more particularly concerns the construction of such contacts or connection to provide compliance or self-adjustment giving intimate contact with plating at a hole through the board, and enhancing reliability.
In the early days of computers, logic wiring was constantly changed, and thus the computer was "programmed" by plug-in wires called "patch cords", patching one component to another. These patch cords were located at the back or "back plane" of the computer. As transistors have advanced, and developed into a plurality of switches (gates, as they are called), logic is programmed into the computer by opening or closing said switches and gates. In this way, actual wiring is not physically disturbed. Subsequently, printed wiring boards carried the logic and memory components, with said wiring boards pluggable into "edge card connectors." Mounted on a "backplane", this is the same technology used to this day. However, the "backplane" still employs a plurality of posts, emanating from the back sides of the edge card connectors. These posts are wired by wire wrapping methods to program the computer, during manufacture. Program changes, and new programs are made by transistor switching. Further development of the "backplane" embodied the introduction of large, thicker printed wiring boards, with interconnecting circuits, to eliminate up to about 75% of wire wrap connections.
There are problems with this approach, for the posts from the connectors have to be soldered to the backplane, and mass wave soldering coats the connector posts with solder, thus making the subsequent wire wrap connections difficult and less reliable. Other mass soldering techniques cause severe warping of the entire backplane, due to high heat. This warpage creates severe reliability problems with the backplane connectors.
Attempts have been made to force-fit solid contact stems into the printed wiring backplane, through the holes previously used for soldered posts. This method works, but the printed-through holes have to be held to very close tolerances. Too large a hole gives a loose pin, with intermittent electrical contact, while a small hole is physically damaged by the tremendous force generated when the pin is forced in. Tight control of hole dimensions is effective, but costly.
One known connector provides a post that adjusts itself to various hole sizes. However it is violently overstressed when pressed into the plated hole, with reliability being about 98% good. The remaining 2% are good until thermal conditions create stress relaxation in the contact material and intermittent contact results (intermittency is the most troublesome fault). Another known compliant device operates like a spring "roll pin". This is effective but is costly to produce, and cannot be produced in close proximity to adjacent contacts, as much raw material is used to produce this device. Other known devices employ through slots in the center of the metal of the compliant section. Extensive testing shows that this approach is even less reliable than the first one, unless one starts to again limit the hole size. Accordingly, there is need for a highly reliable compliant pin, making good contact with plating at all temperatures.